The Homebrewer's Answer Book by Ashton Lewis

Author:Ashton Lewis
Language: eng
Format: epub, mobi, pdf
Publisher: Storey Publishing, LLC
Published: 2007-09-06T04:00:00+00:00

Q What would cause my last few beers from not fermenting all the way to the final gravity the recipe calls for? What factors influence fermentation?

A The final gravity of a beer is an important number to hit. Bottle a beer with residual fermentables, and you are likely to end up with over-carbonated beer and perhaps even bottle grenades. The beer is also likely to taste worty and oversweet. But how do you know where a beer should stop fermenting?

Wort fermentability refers to the percentage of fermentable sugars in wort. Table sugar, unlike wort, contains no unfermentable sugar and is 100 percent fermentable. Wort fermentability typically ranges between 65 and 85 percent. Fermentability depends on three principal variables of the mash: grain types, mash temperature, and mash time.

There is a rather broad range when you look at the finish gravity of beers at the lower and upper ends. For example, a 12° Plato wort (1.048 SG) that is 65 percent fermentable finishes at 4.2° Plato (1.017 SG) compared to 1.8° Plato (1.007 SG) when the fermentability increases to 85 percent. (By the way, I am referring to apparent fermentability gauged using a hydrometer.)

When extracts are used for brewing, you really don’t know how they were made and cannot predict the fermentability of the wort, until you experiment with different brands of extract. Brewers who mash can look at a few key variables to predict how their wort should ferment. Worts with a lot of unfermentable sugars typically contain a high percentage of special malts (like crystal malts) and are made using a single temperature mash from 155 to 158°F (68–70°C). Highly fermentable worts usually contain little or no special malts and are made using multitemperature mashes with extended steps in the temperature range from 140 to 150°F (60–65°C). This is how dry beers (remember Bud Dry and Asahi Super Dry?) and some light beers are made.

One easy method of assessing the fermentability of a wort sample is running a forced fermentation. To do this, take a sample of wort from a batch, pitch a healthy dose of yeast (at double the normal rate), and proceed with a quick, warm fermentation. Most brewers who rely on this method put a magnetic stir bar in a flask and speed things up by using a stir plate to continuously stir the fermentation. At home, you can swirl the fermenter periodically. When signs of fermentation end, measure the finish gravity and you have an indicator of where your actual fermentation should end.

Let’s assume your test shows that the wort has a low fermentability. Extract brewers don’t have much of an alternative other than choosing a different extract. The companies that make extracts make wort the same as all-grain brewers, so the factors affecting fermentability hold true for both. Talk to your local homebrew store about their selection of extracts with respect to fermentability before purchasing.

Or your forced fermentation indicates a high fermentability, but the actual fermentation fails to finish properly. This test result would indicate a problem with the fermentation itself.

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